(1) Field of the Invention
This invention relates generally to DC-DC switch mode voltage converters and relates more specifically to a buck-boost voltage converter having a constant output voltage.
(2) Description of the Prior Art
DC-to-DC converters are important in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries. Such electronic devices often contain several sub-circuits which each require unique voltage levels different from those supplied by the battery (sometimes higher or lower than the battery voltage, or even negative voltage). Additionally, the battery voltage declines as its stored power is drained. DC-to-DC converters offer a method of generating multiple controlled voltages from a single variable battery voltage, thereby saving space instead of using multiple batteries to supply different parts of the device.
Electronic switch-mode DC-to-DC converters are available to convert one DC voltage level to another. These circuits, very similar to a switched-mode power supply, generally perform the conversion by applying a DC voltage across an inductor or transformer for a period of time (usually in the 100 kHz to 5 MHz range) which causes current to flow through it and store energy magnetically, then switching this voltage off and causing the stored energy to be transferred to the voltage output in a controlled manner. By adjusting the ratio of on/off time, the output voltage can be regulated even as the current demand changes. This conversion method is more power efficient (often 80% to 95%) than linear voltage conversion, which must dissipate, unwanted power. This efficiency is beneficial to increasing the running time of battery-operated devices.
A buck-boost converter is a type of DC-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is a switch mode power supply with a similar circuit topology to the boost converter and the buck converter. The output voltage is adjustable based on the duty cycle of the switching transistor. Using a buck-boost converter is especially advantageous with applications wherein a battery provides a voltage, which can be either higher or over time lower than the voltage level required by a load.
Some implementations of buck-boost converters switch between two or three modes when the input voltage changes. With falling input voltage the buck duty cycle rises until the maximum is reached. The buck converter might go into a bypass mode wherein the input and the output are shortened or continues to operate with maximum duty cycle.
FIG. 1a prior art shows the architecture of a prior art buck-boost converter 1. A mode-control block 3 controls, depending upon the level of input voltage Vin, if the converter is operated in buck-mode, in boost mode, or in bypass mode. The mode-control block 3 can shorten the input voltage to the output voltage in the bypass mode via bypass switch 11. The buck-mode is controlled by a buck control block 4, the boost mode is controlled by a boost control block 5. The buck control is performed via buck high-side switch 7 and buck low-side switch 8. The boost control is performed via boost high-side switch 9 and boost low-side switch 10. During a duty cycle the input DC voltage Vin is applied across an inductor L2. Electronic noise of the output voltage Vout, caused by switching at high frequencies, is filtered by capacitor 6.
FIG. 1b prior art shows the output voltage Vout dependent upon the input voltage Vin. In prior art no constant output voltage can be obtained by a buck-boost converter when the input voltage is close to the output voltage, i.e. between a high voltage threshold Vth_hi and a low voltage threshold Vth_low. Both threshold voltages are close to the output voltage desired. During this “interim phase” the buck-boost converter might either go into a bypass mode, wherein the input and the output are shortened, or continue to operate in the buck mode at maximum duty cycle. In both cases no constant output voltage is obtained. As shown in FIG. 1b prior art the output voltage increases first and drops then. The boost-mode will be finally enabled with further dropping of the input voltage.
It is a challenge to the designer of buck-boost converters to achieve converters providing a constant output voltage even during the interim phase.
There are more known patents or patent publications dealing with the design of buck-boost converters.
U.S. patent (U.S. Pat. No. 5,999,419 to Marrero) proposes a non-isolated boost converter with input and output current steering. The input current steering is connected across the input switching transistor, is magnetically coupled to the input inductor and conducts the input current when the switch is turned off, thereby reducing ripple in the input current. The output current steering is connected across the diode in the low-pass output filter and is magnetically coupled to the input inductor to generate an induced current for the output filter when the switch is turned on, thereby preventing the output current from pulsating. With the output filter connected across the switch, a boost converter is formed with the output dc voltage being greater than the input dc voltage in relation to the switching duty cycle. With the output filter connected across the input inductor, a buck-boost converter is formed with the output dc voltage being greater than or less than the input dc voltage in relation to the switching duty cycle.
U.S. patent (U.S. Pat. No. 6,650,095 to Aiello et al.) discloses a converter using the energy stored in the output filter of a step-down (or buck) converter and in the inductor of a step up/down (or buck-boost) converter to supply a second output of opposite sign. In particular, the converter has a first input receiving an input voltage; a first output supplying a first output voltage of a first sign; a second output supplying a second output voltage of opposite sign; a controlled switch connected between the first input and a first intermediate node; an inductor connected between the first intermediate node and the first output; a diode connected between the first intermediate node and a second intermediate node; and a dual voltage generating circuit connected between the second intermediate node and the second output.
U.S. patent (U.S. Pat. No. 6,404,172 to May) discloses a method and apparatus for integrating buck or boost converting including processing for controlling the configuration of transistors, an inductor, a power source, and a load to provide buck or boost converting. Such processing begins by determining whether a buck/boost signal is indicating buck operation or boost operation. If the buck/boost signal is indicating buck operation, the processing proceeds by generating a buck control signal and a load control signal. The buck control signal is provided to the transistors such that the transistors are coupled to a power source and the inductor to provide a buck converter. The load control signal is provided to a load transistor to regulate the output with respect to the load. When the buck-boost signal indicates boost operation, the processing generates a boost control signal and a load control signal. The boost control signal is provided to the transistors such that the transistors are coupled to the power source and the inductor to provide a boost converter. The load control signal is provided to the load transistor to regulate the output of the external load.